The present invention relates to particulate material delivery and screening, but more particularly to a method and an apparatus that cleans and prevents adhesion of fine particles or powders to a mesh during screening operations.
Electrophotographic imaging systems, for example, use color toners and emulsion aggregates in the form of fine particles that represent various colors in the imaging process. To improve image quality, such toner particles require screening to limit the range of particle size for the various imaging stages in the imaging system. Conventional screening is performed by a mesh size in the order of 37 to 74 xcexcm, although standard screen gradations generally vary from about 20 xcexcm or smaller to about 1000 xcexcm. In many advanced electrophotographic machines, however, toner particle size requirements have lessened in size down to a mean diameter of about 5.0 to 8.5 xcexcm, which is obtained by screening through a finer mesh, such as a 20 xcexcm screen mesh. Finer toner particles are difficult to screen through these smaller meshes, particularly if they include chemical additives to improve charging, flow, lubrication, and/or fusing at the various possessing stages of an electrophotographic machine. Such additives have a tendency to adhere to or blind the wire mesh used to screen the particles. Various toner formulations may include as much as 3 to 10% by weight of such additives.
Smaller screen sizes and greater amounts of additives increase screen blinding and impair cleaning. This also retards screening operations thereby lowering the overall efficiency and throughput of the machine, and also causes material waste since good fines may be discarded with coarse material during impaired screening operations. The addition of chemical additives causes toner buildup and particle fusing on the screen mesh, which undesirably impacts the overall life of the screen and downtime of the machine. The life of a turbo screen in Xerox models 5760 and 5790, for example, is about 5000 pounds of toner whereupon the screen must be removed, cleaned, and/or replaced. Some screens have a life of about 10,000 to 12,000 pounds of toner before they require removal and/or solvent cleaning. Imaging is one example in which fine particulate material screening is performed, it being understood that screen life, machine down time, and service cycles varying according to the industrial application at hand.
During turbo screening, for example, fine and coarse particles are separated in a cyclone chamber by weight or particle size where the coarse particles are collected at a periphery of the chamber and discarded. Blinding of the turbo screen during operation causes some of the fine particles to be discarded with the coarse material, thereby increasing the overall yield loss of the fine particles during the screening process. On the other hand, lessening the chance of screen blinding improves overall yield. In an electrophotographic imaging application using resinous toners, this save substantial amounts of color and black fines.
U.S. Pat. No. 5,016,055 assigned to the assignee hereof discloses vibratory energy applied to a charge retentive surface in order to enhance imaging in an electrophotographic imaging process. Numerous other vibratory and ultrasonic systems are described therein, all of which are incorporated herein by reference. Ultrasonic cleaning has also previously been used in electrophotographic imaging machines, such as disclosed in U.S. Pat. No. 5,915,566 by Senapati and assigned to Sweco Incorporated. The Sweco patent describes an ultrasonic ring disposed about the periphery of a screen to improve particle screening in a horizontal gravity fed screener. That system has been shown to clear screen blinding caused by toner particles having a high content of fines, e.g., 60-70% by number of particle sizes between 0.126 xcexcm and 4 xcexcm, in color toners of low melting polyester.
The present invention addresses particle screening and de-blinding in a turbo screening operation that may, for example, be used in an electrophotographic imaging machine. The present invention also addresses design issues that arise in retrofitting existing screeners with improved de-blinding and cleaning systems and methods. The present invention further addresses energy consumption and efficiency issues that arise when attempting to provide ultrasonic screening, generally.
In accordance with the invention, there is provided a particulate delivery system that includes ultrasonic cleaning for a screen where the system comprises a housing, a chamber (e.g., a cyclone chamber) within the housing that separates particles by moving or circulating the particles within the chamber, a screen located within the chamber that passes particles less than a predetermined size, and at least one transducer coupled to the housing that delivers ultrasonic energy to the screen during screening operations whereby to dislodge particles adhering to the screen.
In accordance with another aspect of the invention, de-blinding and screen cleaning is provided for a turbo screener used in an electrophotographic imaging machine. Such an improvement comprises first and second transducer assemblies that each comprise a transducer, a rigid membrane that is ultrasonically driven by the transducer, and a mounting mechanism that supports the transducer assembly on a frame of the turbo screener so as to acoustically couple each membrane and the screen.
In accordance with yet another aspect of the invention, a method of delivering particulate material in a particulate material delivery system comprises circulating or agitating the particle material within a chamber to separate the particulate material by at least one of size and weight, screening the particulate material during within the chamber using a mesh that passes particles of a predetermined size, and inducing ultrasonic waves within the chamber and applying the waves to the mesh during screening whereby to dislodge particles adhering to the mesh.
Advantages provided by the invention include longer screen or mesh service and/or replacement cycles, less downtime, lower yield loss of fines, and the prevention of screen blinding during screening. In addition, the design and construction of the ultrasonic device provided herein conveniently enables retrofitting existing screening devices.
Other features, advantages, and aspects of the invention will become apparent upon review of the succeeding description taken in connection with the accompanying drawings. The invention, though, is pointed out with particularity by the appended claims.